What motor are you going to use? Say it's the HS 142f 49cc. You then get the measurements for that motor and then see if it will fit in the diamond of the bike frame. Also you need to remember a flat plate will be used to mount the motor so the bottom of the diamond isn't ground 0 to measure from. Are you going to use a jackshaft to use the gearing on the bike? Are you just going to use a single sprocket w/o a jackshaft? What transmission are you going to use? You need to figure all this out so it will all fit together to save fabrication issues and money. If you choose the right bike/kit its all plugnplay. All this is assuming you want an inframe kit. Transmissions, well some are junk and some are good. Ask around or read about them, I don't have much experience w/them only what I read. Motors, you can get a honda 35cc but it won't make you happy. It's only good on the flats no hills. The HS 142f/144f are the most popular. The 144f with only 4 more cc'ss has more torque than the 142f and will put a smile on your face and make you happy going up hills etc. Then there's the honda gxh 50cc. Very pricy plus you need a clutch converter to make it work and it sticks way out because of the added width. I'm not going to mention much about the 79cc/212cc motors. That's up to you. If your going to go that big I'd just buy a 150cc scooter or a m/c, my opinion.